1. Field of the Invention
This invention relates to a preform for a light metal alloy composite product mixed with magnesium, a preform suitable as a reinforcement for making the light metal alloy composite product which is produced by sintering oxidized ceramic particles or whiskers and process of forming the preform.
2. Description of Related Art
In order to reinforce an aluminum alloy products mixed with magnesium such as AC4A and AC4AC aluminum alloys (which are specified in Japanese Industrial Standard H5202--Aluminum Alloy Castings 1992), it is typical to combine an aluminum alloy product with a reinforcement such as discontinuous or chopped fibers, ceramic whiskers and/or ceramic particles. In recent years, in view of partly combining an aluminum alloy product with a reinforcement and the simplicity and convenience of composite product formation, it is a tendency to make use of a continuous pore type of pre-shaped porous form (which is referred to as a preform) as a reinforcement for a composite product. This porous preform as a reinforcement is impregnated with a molten aluminum alloy to convert into a composite product. As materials used to make the porous preform, there are enumerated titanium oxide particles which are easily sintered and relatively cheep as ceramic particles, and aluminum borate (9Al.sub.2 O.sub.3.2B.sub.2 O.sub.3) whiskers and potassium titanate (K.sub.2 O.sub..6 TiO.sub.2) whiskers as ceramic whiskers.
In order for a composite product produced from the preform formed by sintering ceramic particles or ceramic whiskers to produce an increase in strength and hardness, it is typical to apply hear treatment to the composite product. However, it is difficult to increase the strength and hardness of a composite product produced by impregnating a magnesium contained aluminum alloy as a base metal with a sintered ceramic preform even by the aid of heat treatment. Table I shows by way of example tensile strength and hardness for products of F- and T6-aluminum alloy castings (AC8A), one of which is mixed with potassium titanate (K.sub.2 O.6TiO.sub.2) whiskers as a reinforcement and the other of which is not mixed with any reinforcement. The symbols "AC8A" and "AC4A" as used in the specification are designations for types of aluminum alloy castings defined in JIS H5202 (Japanese Industrial Standard, 1992) and the symbols "F" and "TX" as used herein are temper designation for aluminum and aluminum alloys defined in JIS H0001 (Japanese Industrial Standard, 1988). The temper designation "F" designates aluminum and aluminum alloys as they are produced with neither work hardening nor heat treatment, and the temper designation "T6" designates aluminum and aluminum alloys which have been provided with heat-treatment and subsequent precipitation hardening treatment for stable mechanical properties.
TABLE I Tensile Strength Molten Metal Casting (MPa) Hardness (Hv) Base T6- T6- Metal Reinforcement F-Product Product F-Product Product AC8A Potassium 200-230 200-230 110-120 11-120 Titanate AC8A None 180 340 85 150
As summarized in Table I, it is apparent on comparison between an F-aluminum alloy (AC8A) product and T6-aluminum alloy (AC8A) product that heat-treatment is effective in increasing tensile strength and hardness of the AC8A aluminum alloy product not mixed with any reinforcement and is however of no effect on the AC8A aluminum alloy product mixed with potassium titanate (K.sub.2 O.6TiO.sub.2) whiskers as a reinforcement. On account of this, there are even cases where aluminum alloy products mixed with no reinforcement outperform reinforcement mixed aluminum alloy products in strength and hardness when they have been heat-treated.
This is thought to arise from the following case. During heat treating a magnesium (Mg) contained aluminum alloy to provide a solution of the aluminum alloy, the magnesium (Mg) chemically combines with silicon (Si) to precipitate magnesium silicate (Mg.sub.2 Si) in the structure with an effect of increasing hardness of the aluminum alloy. However, when an oxide type ceramic used to form a preform contains a compound capable of easily chemically combining with magnesium (Mg), while the preform is transformed into a solution under a high temperature during producing a composite product or during heat treatment of a composite product produced from the preform, the oxide type ceramic chemically combines with magnesium (Mg) with an effect of reducing precipitation of magnesium silicate (Mg.sub.2 Si) which brings about precipitation hardening.
Specifically, in the case where the utilization is made of a titanium oxide or a potassium titanate (K.sub.2 O.6TiO.sub.2) as an oxide type ceramic to form a preform, magnesium has been considered to be is consumed with progress of the reaction expressed by the following chemical formula (I). In this instance, titanium (Ti) precipitated as a result of the chemical reaction expressed by the formula (I) further reacts with aluminum (Al) as expressed by the following chemical formula (II) when an aluminum alloy is used as a base metal. EQU 3TiO.sub.2 +2Mg.fwdarw.2MgTiO.sub.3 +Ti (I) EQU Ti+3Al.fwdarw.Al.sub.3 Ti (II)
The consumption of the magnesium (Mg) deprives the preform of precipitation of magnesium silicate (Mg.sub.2 Si) and, in consequence, there is no increase in hardness of a composite product by means of heat treatment.
When the utilization is made of aluminum borate (9Al.sub.2 O.sub.3.2B.sub.2 O.sub.3) as an oxide type ceramic, the aluminum borate (9Al.sub.2 O.sub.3.2B.sub.2 O.sub.3) and decomposed aluminum oxide (Al.sub.2 O.sub.3) react with magnesium (Mg) to prevent precipitation of magnesium silicate (Mg.sub.2 Si), and hence precipitation hardening.
Further, when the utilization is made of a preform formed by sintering ceramic whiskers, it is hard to provide an increase in hardness by means of heat treatment, and moreover there occurs aggravation of physical properties of a composite product produced from the preform due to disappearance of whiskers resulting from a reaction of the oxide type ceramic with magnesium and/or formation of porosity or cavities due to the disappearance of whiskers.
As described in Japanese Unexamined Patent Publication No. 6-192765, it has been known to add a powdered inorganic binder in ceramic whiskers and/or chopped fibers which is press formed as a preform. The inorganic binder is prepared by cooling a molten mixture of silicon oxide (SiO.sub.2), an aluminum oxide (Al.sub.2 O.sub.3), a magnesium oxide (MgO) and calcium oxide (CaO) and breaking the mixture into powders. During producing the inorganic binder powders, the silicon oxide (SiO.sub.2) possibly reacts with the aluminum oxide (Al.sub.2 O.sub.3), magnesium oxide (MgO) and/or calcium oxide (CaO) to produce chemical compounds and, in consequence, will be unable to afford its own reaction with ceramic whiskers and/or chopped fibers during heat-treating a mixture of the ceramic whiskers and/or chopped fibers and inorganic binder.